Control of a piezoelectric motor

ABSTRACT

A method for controlling a piezoelectric motor, such that control signals of the motor are periodic non-sinusoidal voltage signals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the control of piezo-electric motors.

2. Discussion of the Related Art

Piezoelectric motors such as motors of type piezolegs sold by PiezomotorCompany, are widely used to ensure small displacements of elements ofsmall dimensions. Thus, such motors are used, for example, to ensure theenlargementreduction function (zoom) of a lens of a device of smalldimensions for taking fixed or animated pictures. Such devices areespecially incorporated in battery-supplied portable devices having amain function other than taking pictures such as telephones or personalorganizers.

FIG. 1 schematically and partially illustrates the operating principleof such a motor 1. An axis AXIS to be displaced in a horizontaldirection rests on four pads P1, P2, P3, and P4. Each pad Pi, where i isequal to 1, 2, 3, or 4, rests on a piezoelectric foot Fi. The feet arearranged in two intercalated pairs F1, F3, and F2, F4. Pair F1-F3 iscontrolled by a voltage signal V13. Pair F2-F4 is controlled by avoltage signal V24. Signals V13 and V24 conventionally are sinusoidalsignals of a 6-V amplitude and of a frequency on the order of from 80 to100 kHz. Signals V13 and V24 typically have the same sinusoidal shapes,frequencies and amplitudes, but are phase-shifted, in phase quadrature.

A known method for controlling such a motor comprises the generation ofsinusoidal signals by means of an amplifier directly connected to themotor input. Such a method consumes too high a current for mobileapplications.

Rather than generating sinusoidal signals directly provided to themotor, it has been provided to obtain said signals by filtering adigital signal by means of a resonant LC filter having its capacitiveelements formed by the very motor.

Such a method is described hereafter in relation with FIGS. 2, 3A, and3B.

FIG. 2 schematically and partially illustrates an equivalent dynamicelectric diagram of piezoelectric motor 1 of FIG. 1 associated with aknown control circuit. Pairs F1, F3 and F2, F4 are modeled by twodistinct parallel connections 2 and 3, between the same high and lowsupply rails Vp and GND, of a series association of two respectivecapacitors C1, C3 and C2, C4 and of a series association of tworesistors R1, R3 and R2, R4, respectively. Capacitors C1, C2, C3, and C4represent the equivalent capacitances of motor 1 while resistors R1, R2,R3, and R4 model the losses. Each connection 2, 3 is associated with apulse generator (a digital state machine) respectively 4 (signal S1) or14 (signal S2).

Generator 4 generates a periodic square signal S1 illustrated in FIG.3A. The duty cycle of square wave S1 of FIG. 3A is equal to ½, that is,signal S1 is at a high level for a same time period (half-period) as ata low level. The output of generator 4 is connected to a first terminalof an inductive element, for example, a coil 6, having a second terminalconnected to a common midpoint A1 of series associations C1-C3 and R1-R3of connection 2. Midpoint A1 corresponds to terminals of feet F1 and F3.As a result, signal S1 is filtered by the LC filter formed of element 6and of equivalent capacitors C1 and C3. The filtering is performed toobtain, at point A1, a perfect sinusoidal control voltage signal VA1illustrated in FIG. 3B, of a 6-V amplitude.

Similarly, an inductive element, for example, a coil 16, is interposedbetween generator 14 and a common midpoint A2 of series associationsC2-C4 and R2-R4 of connection 3. Midpoint A2 corresponds to terminals offeet F2 and F4. Digital signal S2 generated by generator 14 has the sameduty cycle as signal S1, but phase-shifted with respect thereto toprovide pair F2, F4 with a sinusoidal control voltage signal VA2 atpoint A2, similar to signal VA1 but in phase quadrature with respect tothe latter.

To obtain sinusoidal control signals VA1 and VA2 of a given nominalfrequency ranging between 80 and 100 kHz, resonant LC filters at thenominal frequency are used. Equivalent capacitances C1, C2, C3, and C4of motor 1 being very low, on the order of 50 nF, this results in usingelements 6 and 16 having a very high inductance, on the order of 30 μH.

A disadvantage of such a method lies in the fact that such elements 6and 16 are very bulky, which is particularly disturbing in applicationsof optical units integrated in battery-supplied portable devices.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method forcontrolling a piezoelectric motor which overcomes all or part of thedisadvantages of known methods.

Another object of the present invention is to provide such a methodwhich uses a low-bulk circuit to be integrated.

Another object of the present invention is to provide such a methodwhich uses a circuit having a decreased power consumption.

To achieve all or part of these and other objects, the present inventionprovides a method for controlling a piezoelectric motor, such thatcontrol signals of the motor are periodic non-sinusoidal voltagesignals.

According to an embodiment of the present invention, each control signalis obtained by applying, to an inductive element, forming withequivalent capacitors a non-resonant LC filter, a periodic signalgenerated by a digital state machine.

According to an embodiment of the present invention, each period of thedigital periodic signal is formed of sub-signals of the same number ofbits, each bit being able to take a state selected from among first andsecond values.

According to an embodiment of the present invention, each period of asignal generated by the state machine comprises at least:

a first series of sub-signals in which a selected quantity of mostsignificant bits of the first value of the sub-signal becomes of thesecond value;

a given integral number of repetitions of a first single-bit signalformed of the bit of the second value;

a second series of sub-signals, inverse to the first series, in which asame selected quantity of least significant bits having the second valueof the preceding sub-signal becomes of the first value; and

the given number of repetitions of a single-bit signal formed of the bitof the first value.

According to an embodiment of the present invention, the sub-signalsampling frequency is 2 MHz, the number of bits being ten, and:

the selected quantity is equal to one;

the number of repetitions ranges between 0 and 25;

the first sub-signal of the first series only comprises bits of thefirst value;

the last sub-signal of the first series only comprises bits of thesecond value;

the first sub-signal of the second series only comprises bit of thesecond value; and

the last sub-signal of the second series only comprises bits of thefirst value.

According to an embodiment of the present invention, the sub-signalsampling frequency is 2 MHz, the number of bits being ten, and:

the selected quantity is greater than one;

the first sub-signal of the first series comprises at least one mostsignificant bit of the second value;

the last sub-signal of the first series comprises at least one leastsignificant bit of the first value and is repeated to reach the minimumhalf-period corresponding to a desired maximum frequency value;

the first sub-signal of the second series comprises at least one leastsignificant bit of the first value; and

the last sub-signal of the second series comprises at least one mostsignificant bit of the second value and is repeated to reach the end ofa minimum period corresponding to a desired maximum frequency value.

The present invention also provides a piezoelectric motor associatedwith a control circuit capable of implementing a method according to anyof the foregoing embodiments to provide two phase-shifted motor controlvoltage signals.

The present invention also provides a photographic lens, comprisingmeans capable of cooperating with a motor according to the precedingembodiment.

The present invention also provides a photographic device comprising alens according to the preceding embodiment.

The present invention also provides a telephone device comprising aphotographic device according to the preceding embodiment.

The foregoing and other objects, features, and advantages of the presentinvention will be discussed in detail in the following non-limitingdescription of specific embodiments in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, previously described, is a partial diagram of a piezoelectricmotor;

FIG. 2, previously described, is a partial equivalent electric diagramof the motor of FIG. 1 and of its conventional control circuit;

FIGS. 3A and 3B, previously described, are timing diagrams schematicallyand partially illustrating the conventional control of the motor of FIG.2;

FIG. 4 is a partial electric diagram of a piezoelectric motor and of itscontrol circuit according to an embodiment of the present invention;

FIG. 5A is a simplified partial timing diagram illustrating a digitalsignal generated by a state machine of the circuit of FIG. 4 accordingto an embodiment of the present invention;

FIG. 5B is a simplified partial timing diagram illustrating a controlsignal of a piezoelectric motor obtained from the digital signal of FIG.5A;

FIG. 6A is a partial simplified timing diagram illustrating a digitalsignal generated by a state machine of the circuit of FIG. 4 accordingto another embodiment of the present invention; and

FIG. 6B is a partial simplified timing diagram illustrating a controlsignal of a piezoelectric motor obtained from the digital signal of FIG.6A.

DETAILED DESCRIPTION

For clarity, the same elements have been designated with the samereference numerals in the different drawings and, further, as usual, thetiming diagrams of FIGS. 3A, 3B, 5A, 5B and 6A, 6B are not drawn toscale.

A feature of an embodiment of the present invention is to control apiezoelectric motor with a non-sinusoidal periodic voltage signal.

FIG. 4 partially and very schematically shows an embodiment of thepresent invention.

For simplification, only the control circuit associated with connection2 (FIG. 2), corresponding to pair F1, F3 of feet of motor 1 (FIG. 1)will be described in detail. Further, the equivalent electric diagram ofthe motor connection, similar to that of FIG. 2, is only partiallyshown.

A digital state machine (DIG) 40 provides a digital signal sent to afirst terminal of an inductive element 42, for example, a coil. Thesecond terminal of element 42 is connected to midpoint A1. Preferably, aclass D amplifier 44 is interposed between machine 40 and element 42 toraise the voltage level of the digital signal, for example, from astandard 3-V level to a 6-V level.

According to a feature of the present invention, the inductance ofinductive element 42 is of small value as compared with that resultingin a resonant LC filter at the frequency of the received signal. The LCfilter formed by the association of element 42 and of capacitors C1 andC3 of motor 1 exhibits, according to the present invention, a resonancefrequency different from the frequency of the received digital signal.For example, for a digital signal of a frequency ranging between 80 and100 kHz, considering that the substantially equal values of equivalentcapacitances C1, C2, C3, C4 are on the order of from 50 to 75 nF, theinductance of element 42 will range between 2 and 3 μH, for example, 2.2μH.

The digital signal output by machine 40 is filtered by the nownon-resonant LC filter 42-C1, C3 to provide at point A1 the motorcontrol voltage signal.

Similarly, the control circuit (not shown) associated with a pair F2, F4of FIG. 1—connection 3, FIG. 2—comprises a digital state machine, acoil, and a class-D amplifier arranged in a same way as the elements ofthe control circuit of connection 2. However, the digital signal ismodified to obtain at point A2 a voltage control signal V24phase-shifted with respect to the provided signal VA1 obtained at pointA1 with the circuit of FIG. 4.

FIGS. 5A, 5B, 6A, and 6B schematically and partially illustrate varioussignals according to different embodiments of the present inventionmeasured at various points of the circuit of FIG. 4.

For clarity, only the digital signal, FIGS. 5A and 6A, provided bymachine 40 to obtain a signal VA1, FIGS. 5B and 6B, at point A1 isdescribed hereafter. It will be within the abilities of those skilled inthe art, based on the descriptions of such signals, to obtain the samesignal VA2 for feet F2 and F4 of motor 1, but phase-shifted with respectto signal VA1.

FIG. 5A shows a timing diagram of a pulse train S5 provided by circuit40 and FIG. 5B schematically illustrates a period T of signal VA1obtained at point A1 by filtering of digital signal S5 of same period T.

Digital signal S5 is divided—sampled—into sub-signals. Each digitalsub-signal comprises a number N of pulses or bits.

Signal S5 is described hereafter in the case of a ten-bit sampling, butshown for simplification with a sampling of four bits only.

A first half-period (T/2) of signal S5 starts with a single initialsub-signal only formed of bits “0”. Then, the most significant bit equalto “0” is replaced with a bit “1”. Such a replacement is performed bystep of one to reach a first central sub-signal only formed of bits “1”.

In the case of a ten-bit sampling, the first half-period of digitalsignal S5 is formed of the following sequence: 0000000000; 1000000000;1100000000; 1110000000; 1111000000; 1111100000; 1111110000; 1111111000;1111111100; 1111111110; 1111111111.

In the shown example of a four-bit sampling, signal S5 thus starts witha sequence: 0000; 1000; 1100; 1110; 1111.

As illustrated in FIG. 5B, such a first half-period of signal S5 leads,after non-resonant LC filtering 42-C1, C3, to a first half-period ofcontrol signal VA1 increasing from a minimum value to a maximum valueVdd. The interval between the minimum and maximum values is six volts.For example, the minimum value is zero while the maximum value is equalto 6 V.

Then, during the next half-period, digital signal S5 repeats once thecentral sub-signal only comprising “1”s and returns to a finalsub-signal only comprising “0”s by replacing, by steps of one, the leastsignificant bit “1” with a bit “0”.

In the case of a ten-bit sampling, signal S5 ends with a sequence:1111111111; 1111111110; 1111111100; 1111111000; 1111110000; 1111100000;1111000000; 1110000000; 1100000000; 1000000000; 0000000000.

In the example shown in FIG. 5A of a four-bit sampling, signal S5 thusends with a sequence: 1111; 1110; 1100; 1000; 0000.

Such a second half-period of signal S5 corresponds to a decreasinghalf-period of control signal VA1 illustrated in FIG. 5B.

Signal VA1 is, as illustrated in FIG. 5B, a triangular signal of periodT.

The present inventors have found that, unexpectedly, the operation ofmotor 1 is not affected by the replacing of the conventional sinusoidalcontrol voltage signal of FIG. 3B with the triangular signal of FIG. 5B.

Further, inductive element 42 having a relatively low value, on theorder of 2.2 μH, is advantageously insertable in an optical unit,conversely to the corresponding element 6 of a 30-μH value used in theembodiment of FIG. 2.

Another advantage of such a method lies in its reduced powerconsumption. Thus, while the generation of a sinusoidal signal by thecircuit of FIG. 2 would cause a power consumption on the order of 500mW, the power consumption of the control circuit according to thepresent application is decreased to approximately 50 mW.

To increase the period of voltage signals VA1 and VA2, and thus decreasetheir frequency, from the triangular signal of minimum period—maximumfrequency, for example, of 100 kHz—of FIG. 5B, the peaks at the minimum(0) and maximum (Vdd) end values are replaced with steady states at suchvalues. For a given signal VA1, the duration of stabilization at theminimum value is equal to the duration of stabilization at the maximumvalue.

As compared with the digital circuit of FIG. 5A, the digital signalenabling obtaining such a control signal comprises a same number M ofrepetitions, at the end of the first sequence, of a single-bit signalformed of bit “1” and, at the end of the second sequence, of asingle-bit signal exclusively formed of bit “0”.

Assuming that the sampling of digital signal S5 is 2 MHz and that numberN of bits is 10, to vary the frequency of control signal VA1 between 80and 100 kHz, number M of repetitions of the single-bit signals rangesbetween 1 and 25.

The operation of piezoelectric motor 1 is not affected by the use ascontrol signals of such signals.

In the preceding embodiments, it has been considered that the increasingand decreasing sequences of the digital signal are obtained by replacinga single bit with its complementary value.

According to another embodiment of the present invention, signalsexhibiting faster increasing and decreasing sequences are used tocontrol the piezoelectric motor.

Such an embodiment is described hereafter in relation with the timingdiagrams of FIGS. 6A and 6B.

FIG. 6A shows a timing diagram of a pulse train S6 provided by circuit40 and FIG. 6B schematically illustrates a period T of the correspondingsignal VA1 obtained at point A1 by filtering of digital signal S6 ofsame period T.

Signal S6 is described hereafter in the case of a ten-bit sampling, butshown for simplification with a four-bit sampling only.

The period of the signals of FIGS. 6A, 6B is the same as that of thesignals of FIGS. 5A, 5B. For simplification, only the differencesbetween FIGS. 5A and 6A, 5B and 6B are described hereafter.

Signal S6 comprises a first sequence starting from an initial signal,after which a given number P of most significant bits “0” are replaced,by steps of one, with bits “1” until a state in which less than P leastsignificant bits are equal to “0” is reached, where P is greater than 1.

For example, for P=2, in the case of a four-bit sampling illustrated inFIG. 6A, signal S6 comprises the first sequence: 0000, 1100, 1111.Signal S6 thus comprises an increasing sequence with three sub-signalsinstead of five in signal S5 of FIG. 5A.

In the case of a ten-bit sampling, the increasing sequence of digitalsignal S6 will comprise less sub-signals than the homologous signal S5.In practice, the inventors' studies have shown that in this case, it isbetter to start from sub-signal 1000000000. The increasing sequence ofsignal S6 for P=2 thus is: 10000000000; 1110000000; 1111100000;1111111000; 1111111110, that is, five sub-signals instead of eleven forthe sequence of the corresponding signal S5.

To obtain a same maximum frequency, minimum period, the rest of theincreasing sequence of signal S5 is replaced in signal S6 with arepetition of the last sub-signal of the increasing sequence.

Thus, in the example where P=2, in the illustrated case of a four-bitsampling, signal 1111 is repeated twice. In the case of a ten-bitsampling, with a sub-sampling at 2 MHz, for a maximum 100-kHz frequency,signal 1111111110 is repeated six times.

As illustrated by the comparison of FIGS. 5B and 6B, a signal VA1exhibiting a faster increased phase between a minimum value, forexample, zero, and a maximum value Vdd, for example, equal to 6 V,followed by a stabilization period at maximum value Vdd which maintainsuntil the half-period, is thus obtained in FIG. 6B.

Then, signal S6 comprises sub-signals in which the P least significantbits equal to “1” are replaced with zero bits until a sub-signalcomprising less than P bits equal to “1” is obtained.

In the case where P=2, for a four-bit sampling, the sequence then is:1111, 1100, 0000. As compared with signal S5 of FIG. 5A, the rest of thehalf-period is completed by the repetition of twice the last sub-signal0000.

In the case of a ten-bit sampling sub-sampled at 2 MHz, to obtain amaximum 100 kHz frequency, the sequence is: 1111111110; 1111111000;1111100000; 1110000000; 10000000000; followed by a repetition of sixtimes sub-signal 10000000000.

As for the signal of FIG. 5A, to increase the period and thus decreasethe frequency of control signal VA1 with fast increasing and decreasingramps, to enable a frequency variation within the range from 80 to 100kHz, a repetition of from 1 to 25 times of signals formed of a singlebit “0” or “1” is introduced similarly to what has been previouslydescribed.

The operation of the piezoelectric motor is not affected by the use ascontrol signals of a signal VA1 of FIG. 6A and of a similar butphase-shifted signal VA2.

More generally, the base signal having the highest frequency is obtainedfrom a digital signal comprising N-bit sub-sampled sub-signalscomprising:

a first sequence of successive sub-signals in which the P mostsignificant bits equal to “0” are replaced with bits “1”, where P is anon-zero integer smaller than N;

a repetition of the last sub-signal of the preceding sequence, to reachthe end of the first half-period;

a second sequence of successive sub-signals, inverse to the first one,in which the P least significant bits equal to “1” are replaced withbits “0”; and

a repetition of the last sub-signal of the preceding sequence, to reachthe period.

The signal period is then increased, that is, the frequency isdecreased, by repeating a same number of times:

a single-bit signal exclusively formed of bit “1” before the secondsequence; and

a single-bit signal exclusively formed of bit “0” after the repetitionof the last sub-signal which follows the second sequence.

Of course, the present invention is likely to have various alterations,modifications, and improvements which will readily occur to thoseskilled in the art. In particular, the present invention has beendescribed in FIG. 4 with a digital state machine capable of generatingany one of the previously-described digital signals. However, it shouldbe understood by those skilled in the art that according to theconsidered application, the state machine may be designed to generateseveral of these signals.

Further, it will be within the abilities of those skilled in the art toselect the motor control signal according to the shape of the variationof the desired force developed by the motor.

It will also be within the abilities of those skilled in the art toadapt the control signals and the digital signals to the application, inparticular to the desired frequency range. Thus, it will be within theabilities of those skilled in the art to adapt the number of bits andthe number of sub-signals of the different digital signals to a wantedfrequency range.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andthe scope of the present invention. Accordingly, the foregoingdescription is by way of example only and is not intended to belimiting. The present invention is limited only as defined in thefollowing claims and the equivalents thereto.

1. A method for controlling a piezoelectric motor, wherein controlsignals of the motor are periodic non-sinusoidal voltage signals, eachcontrol signal being obtained by applying to an inductive element,forming with equivalent capacitors a non-resonant LC filter, a periodicsignal generated by a digital state machine, and each period of thedigital periodic signal being formed of sub-signals of same number ofbits, each bit being able to take a state selected from among first andsecond values.
 2. The method of claim 1, wherein each period of a signalgenerated by the state machine comprises at least: a first series ofsub-signals in which a selected quantity of most significant bits of thefirst value of the sub-signal becomes of the second value; a givenintegral number of repetitions of a first single-bit signal formed ofthe bit of the second value; a second series of sub-signals, inverse tothe first series, in which a same selected quantity of least significantbits having the second value of the preceding sub-signal becomes of thefirst value; and the given number of repetitions of a single-bit signalformed of the bit of the first value.
 3. The method of claim 2, whereinthe sub-signal sampling frequency is 2 MHz, the number of bits beingten, and wherein: the selected quantity is equal to one; the number ofrepetitions ranges between 0 and 25; the first sub-signal of the firstseries only comprises bits of the first value; the last sub-signal ofthe first series only comprises bits of the second value; the firstsub-signal of the second series only comprises bit of the second value;and the last sub-signal of the second series only comprises bits of thefirst value.
 4. The method of claim 2, wherein the sub-signal samplingfrequency is 2 MHz, the number of bits being ten, and wherein: theselected quantity is greater than one; the first sub-signal of the firstseries comprises at least one most significant bit of the second value;the last sub-signal of the first series comprises at least one leastsignificant bit of the first value and is repeated to reach the minimumhalf-period corresponding to a desired maximum frequency value; thefirst sub-signal of the second series comprises at least one leastsignificant bit of the first value; and the last sub-signal of thesecond series comprises at least one most significant bit of the secondvalue and is repeated to reach the end of a minimum period correspondingto a desired maximum frequency value.
 5. A piezoelectric motor,associated with a control circuit capable of implementing the method ofclaim 1 to provide two phase-shifted motor control voltage signals.
 6. Aphotographic lens, comprising means capable of cooperating with themotor of claim
 5. 7. A photographic device, comprising the lens of claim6.
 8. A telephone device, comprising the photographic device of claim 7.